1. Field of the Invention
The invention relates generally to a respiratory ventilator system. More particularly, the invention relates to a system and method for adaptive high frequency flow interrupter control in a respiratory ventilator system.
2. Description of Related Art
Conventional Mechanical Ventilators (CMV) are commonly used to provide breathing gas to hospital patients. These ventilators can be programmed to control the flow of the breathing gas inhaled and exhaled by the patient. The ventilation operating frequency used for CMV can range from 1-150 BPM, and the tidal volume can range from 4-20 ml/kg.
The breathing gas supplied to patients can be Oxygen, Air, Nitrox or Heliox. Nitrox is a breathing gas consisting of Oxygen (O2) and Nitrogen (N2) that is similar to air, but with a higher proportion of Oxygen than the normal 20.9%. Heliox is a breathing gas that is composed of a mixture of Helium (He) and Oxygen (O2). Since Helium is less dense than Nitrogen, airway resistance is reduced due to the lower density of the inspired gas. This means that when one breathes Heliox, airway resistance is less, and therefore the mechanical energy required to ventilate the lungs, or the Work of Breathing (WOB) is decreased.
One type of respiratory ventilators commonly used for critically ill or neonatal patients is a High Frequency Ventilator (HFV). The HFV utilizes a ventilation technique that delivers very small tidal volumes at a high rate that exceeds the rate of normal breathing. The delivered small tidal volume is usually less than or equal to anatomical dead space volume. The frequency can range from 2.5-15 Hz, preferably no less than 150 Breaths Per Minute (BPM), and the low tidal volume can range from 0.5-5 ml/kg.
A High Frequency Flow Interrupter (HFFI) is a unique subset of an HFV system, where tidal volumes are delivered at high frequencies by “interruption” of delivered flow. HFFIs can be adapted from CMVs, with both conventional and high-frequency options combined. To produce high-frequency breaths, a flow of gas is interrupted by a valve mechanism that may be controlled pneumatically, mechanically or electronically by a microprocessor. Operator-selected parameters typically include mean airway pressure, amplitude pressure, and frequency, which is usually set between 2.5 and 15 Hz.
One HFFI system developed by Infrasonics is Infant Star 950, which “interrupts” the delivered flow by using electronically controlled solenoids to cause pressure pulses. This system utilizes an “open-loop” approach, which requires a clinician to select appropriate settings of mean pressure, frequency and amplitude. The pressure waveform can be manipulated by a series of pneumatic valves. Hence, for example, if the clinician determines that the initial settings were too high for the patient to tolerate or the patient dynamics has changed, the clinician would need to adjust the settings so that a comfortable setting is achieved. To ascertain a comfortable setting for the patient, an iterative manual process is required. This process is often left for the clinician who may not be at the patient bedside as conditions change. Furthermore, the process is prone to inaccuracies and errors.
With an increasing demand for improved HFFIs, there remains a continuing need in the art for an adaptive HFFI system that can provide consistent ventilation performance by automatically compensating for changes in patient dynamics and pressure regulation requirements.